As wells are drilled to greater lengths and depths, it becomes necessary to provide a liner (“casing”) to avoid undesirable fluid inflows or outflows and to prevent borehole collapse. The annular space between the borehole wall and the liner is usually filled with cement (a process referred to as “cementing” the well) to reinforce structural integrity and to prevent fluid flows along the outside of the liner. If such fluid flows are not prevented, there is a loss of zonal isolation. Fluids from high-pressured formations can enter the borehole and travel along the outside of the casing to invade lower-pressured formations, or possibly exit the borehole in a mixture that dilutes the desired production fluid. Results may include contamination of aquifers, damage to the hydrocarbon reservoir, and loss of well profitability.
When cementing a well, the cement is generally injected down the interior of the casing to the bottom of the borehole and forced back upward around the casing. Spacer fluids are first injected into the casing ahead of the cement to separate, and thus reduce contact and mixing between, wellbore fluids (e.g., drilling fluid and cement). After spacer fluid is pumped in, the cement is pumped into the casing. This forces fluid already in the borehole (e.g., spacer and drilling fluid) back up into the annular region between the casing and the formation and to the surface where it is safely collected. Once the desired amount of cement has been injected into the casing, spacer fluid is again injected to force the cement out of the casing, out into the bottom of the borehole and back up the annular region outside the casing. The spacer fluid continues to be injected downward until all of the cement is forced out of the bottom end of the casing. The cement outside the casing is then left to cure before any further drilling or production activities continue.
As cementing proceeds it is useful to monitor the characteristics and flow of the cement as it is introduced into the annular region outside the casing. A number of electromagnetic techniques may be used to monitor such fluid characteristics as composition, density and thickness, just to name a few. Further, radio frequency identification (RFID) tags entrained in the fluids (cement, spacer fluids, etc.) can be used to track the flow rate, flow direction and location of the fluids within the borehole. However, different measurement and tracking techniques may each require different sets of antennas positioned on the exterior surface of the casing. Further, each set of antennas sometimes requires multiple antennas, each with a corresponding RF coaxial cable that connects the antenna to the electronics driving, receiving and processing antenna signals. These electronics are typically encased in a protective housing or “sensor tube” that shields the electronics from the harsh environment present downhole. But as the number of antennas increases, so does the number of cables entering/exiting the sensor tube, requiring an increase in the diameter of the tube to the point where the tube may undesirably interfere with the cement flow. Such interference can result in voids in the cement and/or a reduced cement thickness that may compromise the long-term strength and integrity of the cement after it cures.
It should be understood that the drawings and corresponding detailed description do not limit the disclosure, but on the contrary, they provide the foundation for understanding all modifications, equivalents, and alternatives falling within the scope of the appended claims.